We have developed a method for extracellular labeling of growing peptide chains in bacteria, and have thereby unequivocally demonstrated the secretion of a growing chain across a membrane. We wish to use this method, and also to observe synthesis by membrane-associated polysomes or by free polysomes in vitro, to determine how generally the synthesis of secreted proteins (including bacterial toxins), and the insertion of membrane proteins, proceeds in this manner. The most challenging question--the mechanism of transport of the chain across the membrane--will be studied by the use of mutants altered in secretion, cells with altered lipid composition, a search for an energy requirement, and cross-linking of protruding growing chains to adajacent membrane proteins.